The demand for lithium ion secondary batteries having compact size, light weight, high energy density, and the ability to be repeatedly charged and discharged is expected to increase in the future also from an environmental perspective. Lithium ion secondary batteries are used in various fields such as mobile telephones and notebook computers due to their high energy density. However, expansion of applications for lithium ion secondary batteries and development thereof have been accompanied by demand for even higher performance in terms of low resistance, high capacity, and so forth.
A separator has an important function of preventing an electrical short-circuit between the positive and negative electrodes of a lithium ion secondary battery. The separator of a lithium ion secondary battery is typically a microporous membrane formed, for example, from a polyolefin resin. The separator normally also has a role of maintaining the safety of the lithium ion secondary battery by displaying a shutdown function of cutting off current in a situation in which the battery internal temperature reaches a high temperature of around 130° C., for example, by melting to block the micropores and thereby prevent migration of lithium ions. However, if the battery temperature further exceeds the melting point of the constituent resin of the separator due to momentary heat generation, the separator may rapidly contract, leading to an increase in locations at which the positive electrode and the negative electrode come into direct contact to cause a short-circuit. In such a situation, the battery temperature rises to hundreds of degrees Celsius or higher and the battery reaches an abnormally heated state.
In view of the above, it has been proposed that thermally expansive microcapsules may be used to inhibit a rise in temperature during abnormal heating. PTL 1 discloses a current collector including a resin layer and an adhesive layer that is conductive. Thermally expansive microcapsules in which an expansive material is encapsulated in a shell containing a thermoplastic polymer material are used in the adhesive layer. In another example described in PTL 2, thermally expansive microcapsules in which a low boiling point hydrocarbon is encapsulated in a copolymer shell are dispersed in an electrode active material layer. In yet another example described in PTL 3, thermally expansive microcapsules obtained through in situ polymerization of a low boiling point hydrocarbon or a foaming agent in a thermoplastic resin of vinylidene chloride, acrylonitrile, or the like are added into an electrolysis solution.